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     lseek — reposition read/write file offset


     Standard C Library (libc, -lc)


     #include <unistd.h>

     lseek(int fildes, off_t offset, int whence);


     The lseek() system call repositions the offset of the file descriptor
     fildes to the argument offset according to the directive whence.  The
     argument fildes must be an open file descriptor.  The lseek() system call
     repositions the file position pointer associated with the file descriptor
     fildes as follows:

           If whence is SEEK_SET, the offset is set to offset bytes.

           If whence is SEEK_CUR, the offset is set to its current location
           plus offset bytes.

           If whence is SEEK_END, the offset is set to the size of the file
           plus offset bytes.

           If whence is SEEK_HOLE, the offset of the start of the next hole
           greater than or equal to the supplied offset is returned.  The
           definition of a hole is provided below.

           If whence is SEEK_DATA, the offset is set to the start of the next
           non-hole file region greater than or equal to the supplied offset.

     The lseek() system call allows the file offset to be set beyond the end
     of the existing end-of-file of the file.  If data is later written at
     this point, subsequent reads of the data in the gap return bytes of zeros
     (until data is actually written into the gap).

     Some devices are incapable of seeking.  The value of the pointer
     associated with such a device is undefined.

     A "hole" is defined as a contiguous range of bytes in a file, all having
     the value of zero, but not all zeros in a file are guaranteed to be
     represented as holes returned with SEEK_HOLE.  File systems are allowed
     to expose ranges of zeros with SEEK_HOLE, but not required to.
     Applications can use SEEK_HOLE to optimise their behavior for ranges of
     zeros, but must not depend on it to find all such ranges in a file.  The
     existence of a hole at the end of every data region allows for easy
     programming and implies that a virtual hole exists at the end of the
     file.  Applications should use fpathconf(_PC_MIN_HOLE_SIZE) or
     pathconf(_PC_MIN_HOLE_SIZE) to determine if a file system supports
     SEEK_HOLE.  See pathconf(2).

     For file systems that do not supply information about holes, the file
     will be represented as one entire data region.


     Upon successful completion, lseek() returns the resulting offset location
     as measured in bytes from the beginning of the file.  Otherwise, a value
     of -1 is returned and errno is set to indicate the error.


     The lseek() system call will fail and the file position pointer will
     remain unchanged if:

     [EBADF]            The fildes argument is not an open file descriptor.

     [EINVAL]           The whence argument is not a proper value or the
                        resulting file offset would be negative for a non-
                        character special file.

     [ENXIO]            For SEEK_DATA, there are no more data regions past the
                        supplied offset.  For SEEK_HOLE, there are no more
                        holes past the supplied offset.

     [EOVERFLOW]        The resulting file offset would be a value which
                        cannot be represented correctly in an object of type

     [ESPIPE]           The fildes argument is associated with a pipe, socket,
                        or FIFO.


     dup(2), open(2), pathconf(2)


     The lseek() system call is expected to conform to ISO/IEC 9945-1:1990


     The lseek() function appeared in Version 7 AT&T UNIX.


     This document's use of whence is incorrect English, but is maintained for
     historical reasons.